1. Field of the Invention
This invention relates generally to a video camera and, more particularly, to apparatus for correcting black level values in a video camera.
2. Description of the Background
It is known that all video cameras, regardless of the particular type of image pick-up device employed, produce a video signal even when there is no light incident thereon. This video output level with no incident light is known as the black level value and is due to the so-called dark current of the photosensitive device. Because such video output is present in the absence of any light falling on the image pick-up device, it is necessary to calibrate the video camera by providing some cancelling signal to cancel out the dark current present in that individual camera. This is necessary, of course, because otherwise there would be a video signal output level even though no light was being detected. Furthermore, because it is known that the video output level will vary in relation to changes in the ambient temperature and fluctuations in the bias current, for example, system calibration and/or black level cancellation must be carried out periodically. Also, if black level cancellation has been performed incompletely in the case of a color video camera of the multi-tube or multiple device kind, then the black balance of the camera will be disturbed. Also, drift in the video amplifier and in the video processor, which are a part of the video camera circuitry, will lead to variations in the black level and/or disturbance of the black balance. Thus, it is necessary to perform a detection operation to learn whether or not the black level cancellation has been completely performed at the final output stage of the video signal processing system.
The video signal processing circuit typically employed with this kind of video camera includes a video amplifier that has a gain which is switchable between two levels according to the degree of brightness of the subject being picked up. This introduces a further problem in black level cancelling because the black level of the output amplifier may vary upon the switching of the gain. Therefore, only if there is no change in black level when the gain is switched from one level to the other can it be determined that virtually complete black level cancellation has taken place. The known systems of this kind all simply make one black level cancellation measurement relative to the video amplifier and provide that cancellation signal regardless of the movement of the gain control switch in the video amplifier.
The approaches known heretofore to achieve black level cancellation have all had various drawbacks, for example, because the black level changes relativelv slowly it is known to provide a fixed time delay before performing the black level correction operation. That is, a rather lengthy time delay is imposed to permit the black level to become stabilized before making the appropriate measurements in order to provide the cancellation signal. Nevertheless, this has not been totally suitable because the black level will frequently remain unstable even after the time delay has elapsed and, thus, correction becomes inaccurate. In other words, a fixed time delay is not always appropriate because stabilization does not always occur within the predetermined time or it might occur quite rapidly and the delay time slows down the process. Furthermore, the transfer characteristic of the video signal processing circuit is generally a complex nonlinear characteristic and a complete black level cancellation cannot be obtained by one single cancellation and several black level corrections must be made through switching of the gain of the video amplifier until a converged state is reached. Obviously, this kind of calibration or correction procedure requires a long time. In addition, the nonlinear characteristic of the video signal processor typically employs a dead zone, and the proper black level cancellation signal cannot be obtained in close proximity to the real black level because of this dead zone and, thus, black level cancellation is incomplete.